pointer in a class

[mickey]

Hello,
I'm wondering when is necessary/better declare a pointer to an object in a class or a pointer to a vector in a class

class A {
  public:
   vector<flaoat> fvec;
   //or
  vector<float>* fpvec;
  A() {  
      fvec.resize(1000);
      //or
      fpvec = new vector<float>(1000);
  }
};

class A {
  public:
   vector<flaoat> fvec;
   //or
  vector<float>* fpvec;
  A() {  
      fvec.resize(1000);
      //or
      fpvec = new vector<float>(1000);
  }
};

To copy to clipboard, switch view to plain text mode 

What's the thing that lead me to opt one or other?
THe same question if I had a Person p[10]; in the class. Should be better take a pointer to an array of person? How to choice?

thanks

[wysota]

I'm wondering when is necessary/better declare a pointer to an object in a class or a pointer to a vector in a class

The only reasons to declare a pointer instead of an object I see are:
1. The object pointed by the pointer is not owned by the containing object or the instance is shared among more than one containing object.
2. One wants to only forward declare a class in the header file to speed up compilation of dependent objects and slow down initialization of objects.
3. The platform the application is running on has limitations on the stack size and as much data as possible is pushed onto the heap.

Apart from those three situations I see no reason to use pointers instead of objects.

[Shadowfiend]

Although if you do decide not to use a pointer, it might be a good idea to give the vector its initial size in the initialization list:

A() : fvec(1000) {}

A() : fvec(1000) {}

To copy to clipboard, switch view to plain text mode 

[wysota]

Oh, I just thought of another reason to use a pointer instead of a full object - when the use of the object is optional and the full object is allocated only when needed - then you'll need a pointer. On the other hand it doesn't make much sense to do that with a vector because it already lazy allocates it data on the heap, so both conditions are fulfilled with using a vector object instead of vector pointer. The difference in memory footprint will be minimal (unless you resize the vector when constructing it - then there is no lazy allocation there).

[mickey]

Is there any change to use forward declaration with vector, like this:

class vector; // ?
 
class A {
    vector<float>* fvec;
.......................
};

class vector; // ?

class A {
    vector<float>* fvec;
.......................
};

To copy to clipboard, switch view to plain text mode 

[wysota]

Your code that depends on class A will compile faster as it will not have to parse vector's header file each time A's header is included. The application will run a bit slower though, as it'll have to allocate the vector at some point.

[mickey]

Your code that depends on class A will compile faster as it will not have to parse vector's header file each time A's header is included.

but I asked how write it (this above doens't compile)

The application will run a bit slower though, as it'll have to allocate the vector at some point.

I guess the cause is around the allocation on the heap...

[Shadowfiend]

I think the forward declaration has to look something like:

template <class T> class vector;

template <class T> class vector;

To copy to clipboard, switch view to plain text mode 

[wysota]

I'd say it should be "class vector<float>;". I don't see a reason to use a pointer to a vector in this situation, though... It's just asking for trouble and a necessity to write more code.

[mickey]

I'd say it should be "class vector<float>;". I don't see a reason to use a pointer to a vector in this situation, though... It's just asking for trouble and a necessity to write more code.

class vector<float>;
 
class Shape {
	const std::vector<float>* _sides;

class vector<float>;

class Shape {
	const std::vector<float>* _sides;

To copy to clipboard, switch view to plain text mode 

It doesn't compile; compile gets many error on the first line (like as I missed the ";").
I know It's unuseful, but I'd like compile it for study....
thanks.

[Shadowfiend]

Probably because you didn't put it in a namespace. Try:

namespace std { class vector<float>; }

namespace std { class vector<float>; }

To copy to clipboard, switch view to plain text mode 

or:

namespace std { template <class T> vector; }

namespace std { template <class T> vector; }

To copy to clipboard, switch view to plain text mode 

[mickey]

nothing to do